See? It’s not so easy! My inclination is to do ash but use rust for the rocky outcroppings on some of the tiles. A red shale planetary crust with a pulverized nuclear ash blowing over it. You’re right that it does look a little like standard desert so I like the idea of the rusty plains but it’s a dark value which may not be the best for a minis set. I’m pondering even still – the consensus from my various FB memberships is Ash so far.
I think you’re right that the wash has to either go on below the highlights or just leave off. There are some tiles with rocky outcroppings and those are going to be red shale, but the main base I think I’ll go with the ochre desert as the main color.